The present invention relates to systems for optically measuring biological parameters, such as glucose concentration for example.
There has been substantial activity in the field of optical measurement of biological analytes. One approach is to make such measurements using noninvasive optical systems that direct an optical signal against the skin of a person and measure optical characteristics of reflected, transmitted or scattered light. Messerschmidt U.S. Pat. No. 5,823,951, Mendelson U.S. Pat. No. 5,277,181, Clarke U.S. Pat. No. 5,222,495, and Yamaguchi U.S. Pat. No. 5,127,406 are examples of such noninvasive measurement systems. A recurring problem with this approach is that the skin of the person absorbs, scatters and otherwise affects optical signals to a substantial degree, thereby interfering with the desired measurement. Interference of this type can vary substantially from person to person, and such variations can make it difficult or impossible to achieve high accuracy noninvasive optical measurements.
In principle, it would be possible to implant an entire optical measuring system, thereby eliminating adverse effects of the skin on the optical measurement. See Van Heuvelen U.S. Pat. No. 4,704,029. However, such an implantable system would require power for operation which would represent yet another disadvantage.
Various approaches have been suggested for combining an external optical source and sensor with an implantable optical system. See, for example, Rao U.S. Pat. No. 5,628,310, Palti U.S. Pat. No. 5,368,028, and Slate U.S. Pat. No. 5,605,152. The system described in the Slate patent overcomes the problem of skin interference by using a fiber optic cable to interconnect the internal and external portions of the system. The devices disclosed in the Rau and PaIti patents suffer from the disadvantage that variations in the optical characteristics of the skin can adversely effect the optical measurement.
Thus, a need presently exists for an improved implantable optical system that reduces or eliminates measurement problems associated with variations in the optical characteristics of the skin.